The present invention relates generally to an apparatus and method for transmitting audio signals and pertains, more specifically, to an apparatus and method for transmitting a high quality audio signal, such as wideband speech, through a transmission channel having a limited bandwidth or transmission rate.
Human speech lies in the frequency range of approximately 7 Hz to 10 kHz. Because traditional telephone systems only provide for the transmission of analog audio signals in the range of about 300 Hz to 3400 Hz or a bandwidth of about 3 kHz (narrowband speech), certain characteristics of a speaker""s voice are lost and the voice sounds somewhat muffled. A telephone system capable of transmitting an audio signal approaching the quality of face-to-face speech requires a bandwidth of about 6 kHz (wideband speech).
Known digital transmission systems are capable of transmitting wideband speech audio signals. However, in order to produce an output audio signal of acceptable quality with a bandwidth of 6 kHz, these digital systems require a transmission channel with a transmission rate that exceeds the capacity of traditional telephone lines. A digital system transmits audio signals by coding an input audio signal into a digital signal made up of a sequence of binary numbers or bits, transmitting the digital signal through a transmission channel, and decoding the digital signal to produce an output audio signal. During the coding process the digital signal is reduced or compressed to minimize the necessary transmission rate of the signal. One known method for compressing wideband speech is disclosed in Recommendation G.722 (CCITT, 1988). A system using the compression method described in G.722 still requires a transmission rate of at least 48 kbit/s to produce wideband speech of an acceptable quality.
Because the maximum transmission rate over traditional telephone lines is 28.8 kbit/s using the most advanced modem technology, alternative transmission channels such as satellite or fiber optics would have to be used with an audio transmission system employing the data compression method disclosed in G.722. Use of these alternative transmission channels is both expensive and inconvenient due to their limited availability. While fiber optic lines are available, traditional copper telephone lines now account for an overwhelming majority of existing lines and it is unlikely that this balance will change anytime in the near future. A digital phone system capable of transmitting wideband speech over existing transmission rate limited telephone phone lines is therefore highly desirable.
The disclosed invention has various embodiments that achieve one or more of the following features or objects:
An object of the present invention is to provide for the transmission of high quality wideband speech over existing telephone networks,
A further object of the present invention is to provide for the transmission of high quality audio signals in the range of 20 Hz to at least 5,500 Hz over existing telephone networks.
A still further object of the present invention is to accomplish data compression on wideband speech signals to produce a transmission rate of 28.8 kbit/s or less without significant loss of audio quality.
A still further object of the present invention is to provide a device which allows a user to transmit and receive high quality wideband speech and audio over existing telephone networks.
A still further object of the present invention is to provide a portable device which is convenient to use and allows ease of connection to existing telephone networks.
A still further object of the present invention is to provide a device which is economical to manufacture.
A still further object of the present invention is to provide easy and flexible programmability.
In accordance with the present invention, the disadvantages of the prior art have been overcome by providing a digital audio transmitter system capable of transmitting high quality, wideband speech over a transmission channel with a limited bandwidth such as a traditional telephone line.
More particularly, the digital audio transmitter system of the present invention includes a coder for coding an input audio signal to a digital signal having a transmission rate that does not exceed the maximum allowable transmission rate for traditional telephone lines and a decoder for decoding the digital signal to provide an output audio signal with an audio bandwidth of wideband speech. A coder and a decoder may be provided in a single device to allow two-way communication between multiple devices. A device containing a coder and a decoder is commonly referred to as a CODEC (COder/DECoder).
These and other objects, advantages and novel features of the present invention, as well as details of an illustrative embodiment thereof, will be more fully understood from the following description and from the drawings.